As the level of integration of semiconductor devices using silicon substrates increases, the p-n junction leakage current must be reduced. One factor which contributes to the leakage current is the amount of impurities of heavy metals (e.g. iron, nickel, and copper) which are induced during manufacturing process of the semiconductor devices. In order to remove the heavy metal impurities from the operating region of a semiconductor device and capture the impurities, various gettering methods have been used.
In one gettering method, a polysilicon film is formed on the rear face of a silicon substrate having a first main surface and a second main surface (i.e. the rear face) which opposes the first main surface. By forming the polysilicon film on the rear face of the silicon substrate, the polysilicon film absorbs various impurities from the silicon substrate. Afterwards, a semiconductor device is formed on the first main surface of the silicon substrate, and the impurities which have been absorbed by the polysilicon film do not diffuse towards and contaminate the operating region of the semiconductor device. An example of such method is disclosed in Japanese Patent Laid-Open Sho 59-186331.
The above gettering method was presumed to be effective even after applying a high temperature heat treatment to the semiconductor substrate when a semiconductor device is formed on the substrate. Recently, however, it has become known that the contaminating elements which have been previously gettered by the polysilicon film are released from the film and diffuse io into the operating region of the semiconductor device when the device is heated to 1000.degree. C. or higher and that the polysilicon is subsequently recrystallized during the high temperature heat treatment at 1200.degree. C. When the polysilicon film recrystallizes, its gettering capabilities are eliminated, and thus, it cannot re-absorb the impurities which have been released. Therefore, the advantages obtained from the initial gettering process are subsequently lost.
In order to overcome such problem, several methods have been developed to prevent the polysilicon film from recrystallizing during the manufacturing process of a semiconductor device. One such method is disclosed in Japanese Patent Laid-Open Hei 5-286795 and suppresses the recrystallization of polysilicon by creating a silicon oxide film between the polysilicon film and the silicon substrate. However, although various metals (e.g. copper) can permeate through the silicon oxide film and can be absorbed by the polysilicon film, other metals (e.g. iron) cannot easily permeate through the silicon oxide film to reach the polysilicon film. Therefore, the polysilicon film cannot adequately getter the iron impurities, and such impurities remain in the silicon substrate while a semiconductor device is manufactured. As a result, the impurities diffuse towards and contaminate the operating region of the semiconductor device.
Japanese Patent Laid-Open Hei 1-235242 describes another method for suppressing the recrystallization of polysilicon by ion implanting impurities (e.g. nitrogen, oxygen, or argon) into the silicon substrate before forming the polysilicon film on the substrate. Although this method can suppress the recrystallization of polysilicon, no heat treatment is performed on the impurities that are implanted into the silicon substrate. As a result, no clusters or dislocations are formed from the implanted impurities in the silicon substrate, and thus, the method cannot prevent the contaminating elements and heavy metals which have previously been gettered from being released again into the operating region of the semiconductor device during the manufacture of the semiconductor device.
In Japanese Patent Laid-Open Hei 6-140410, a method for gettering heavy metals has been proposed in which the polysilicon film is removed before the contaminating elements are re-released into the operating region of the semiconductor device and before the polysilicon is recrystallized. For example, the polysilicon film is formed on the semiconductor substrate, various impurities are gettered, and the polysilicon film is removed before the impurities are released back into the silicon substrate. During a subsequent step of the manufacturing process of a semiconductor device, another polysilicon film is formed on the semiconductor substrate, various impurities are gettered, and the polysilicon film is removed before the impurities are released back into the silicon substrate. Such process is repeated several times while the semiconductor device is being manufactured.
However, heavy metal impurities may possibly intrude into the operating region between the first stage and the last stage of the manufacturing process of the semiconductor device. Therefore, since the gettering capabilities of the polysilicon film are lost when the film is removed, the characteristics of the semiconductor device may be degraded if impurities are present in the silicon substrate during the periods when the film is not present on the substrate. Also, the repeated formation and removal of polysilicon film on the silicon substrate increase the time and cost of manufacturing a semiconductor device.